Cutting tip and method thereof

ABSTRACT

A cutting tip having a three-layer laminated structure is provided. The tip is made from a disc-shaped three-layer laminate wherein a second layer consisting of a hard sintered compact of CBN or diamond is sandwiched by a first layer and a third layer consisting of a tool material such as cemented carbide. Prismatic blanks of rectangular cross-section are cut out by cutting the three-layer laminate in strips. Semi-completed tips of a desired shape are obtained by cutting up the prismatic blanks. Desired cutting edges are formed on the semi-completed tips to produce completed tips. Because prismatic blanks are obtained from a three-layer laminate and semi-completed tips are cut out from the prismatic blanks like this, the semi-completed tips can be obtained in large numbers, yield is high, and the manufacturing cost of the tip can be reduced.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a cutting tip which can be manufacturedat low cost, and to the manufacture thereof.

[0003] 2. Description of the Related Art

[0004] Generally, cutting tools are made by attaching a hard cutting tipto the end of a tool body. Cutting tip of this kind include for examplethe “Twist Drill” disclosed in Japanese Utility Model Laid-OpenPublication No. HEI-5-63717 and the “Drill” disclosed in JapaneseUtility Model Laid-Open Publication No. HEI-3-117520.

[0005] In the “Twist Drill” of Japanese Utility Model Laid-OpenPublication No. HEI-5-63717, a wedge-shaped very high pressure sinteredtip integrated with a cylindrical body part made from cemented carbideis used as the starting blank of a cutting edge body. This startingblank is fixed to a tool body by a method such as brazing, and the shapeof an edge is formed to complete the cutting tool. However, in this“Twist Drill”, the starting blank is difficult to manufacture and isexpensive. Consequently, as a result of the cost of the cutting tipbeing high, there has been the problem that the cutting tool isexpensive.

[0006] In the “Drill” of Japanese Utility Model Laid-Open PublicationNo. HEI-3-117520, a drill is manufactured by cutting out a pentagonaltip blank from a disc made by surrounding a disc of cemented carbidewith a ring-shaped a hard sintered compact. However, in the manufactureof this drill, from a disc of cemented carbide surrounded by a ring of ahard sintered compact, only from one to a few of the pentagonal tipblanks can be cut out, and the yield is poor. Consequently, not mucheffect of reducing the cost of the cutting tip or the cost of thecutting tool can be expected.

[0007]FIGS. 59A through 59C illustrate a typical thread-cutting methodof related art.

[0008]FIG. 59A: A drill 512 is set in a machine tool 510; the drill 512is aligned with a cored hole 511, and machining of a prepared hole isstarted. The point angle θ of the drill 512 is 120° in general.

[0009]FIG. 59B: Next, the drill 512 is replaced with a tap 514 forthread-cutting. The reference numeral 513 denotes the prepared hole madewith the drill 512, and its depth is D1. The tap 514 is aligned withthis prepared hole 513 and thread-cutting is started.

[0010]FIG. 59C: The reference numeral 515 denotes a female thread formedby the tap 514. The tap 514 is replaced with a chamfering tool 516, andthe entrance of the thread 515 is chamfered with the chamfering tool516. The reference numeral 517 denotes the chamfer. If the length of thethread 515 including this chamfer 517 is written D2, then an unthreadedpart of length (D1-D2) remains.

[0011] In this thread-cutting method of related art, to form the thread515, a drill, a tap and a chamfering tool are necessary; thus the numberof tools required is large, tool supply costs are high and toolmanagement costs are high. Also, it is necessary for tools to beinterchanged during the process from the hole-making to the chamfering.Because of this, the cutting work must be stopped for every tool change,the thread-cutting machining operation is troublesome, and there areproblems of productivity.

[0012] Also, because an unthreaded part of length (D1-D2) isunavoidable, the unthreaded part must be allowed for in the casting, andconsequently there has been the problem that it is not possible to makethe casting thin.

[0013] Next, as the cutting tool for finishing the hole, generally areamer is used, to finish the opened hole in advance exactly and obtaina smooth finished surface at the same time. By machining the hole with areamer, an accurate hole can be obtained. As the procedure for finishinga hole with a reamer, first a small-diameter hole is made with a drill,and then by dragging the wall of the hole with the reamer the holediameter is finished to the required accuracy (for example, dimensionalaccuracy H7 (JIS B 0401).

[0014] Thus, a reamer is necessary to obtain an accurate hole; however,with this finishing method, the number of tools required is large,numerous tools have to be set on and removed from the machine tool, andpreparation is troublesome. Also, hole-finishing with a reamer takestime, and it is necessary for machining with the machine tool to bestopped for the tool change from the drill to the reamer to be carriedout. Consequently, the productivity of the hole-machining is low.

SUMMARY OF THE INVENTION

[0015] It is therefore a first object of the present invention toprovide an inexpensive cutting tip and cutting tool and a method bywhich this cutting tool can be manufactured.

[0016] It is a second object of the invention to provide athread-cutting method with which it is possible to reduce the number oftools required for thread-cutting and to dispense with an unthreadedpart.

[0017] It is a third object of the invention to provide a hole-finishingdrill which allows good productivity.

[0018] According to a first aspect of the present invention, there isprovided a cutting tip having a first layer consisting of a cementedcarbide tool material, a second layer consisting of a hard sinteredcompact of CBN or diamond, and a third layer consisting of a cementedcarbide tool material; the second layer is sandwiched by the first layerand the third layer to form a three-layer laminate, and a cutting edgeis formed in the second layer.

[0019] Because the cutting tip as a whole is a three-layer structure,and the second layer is reinforced on both sides by the first layer andthe third layer, the rigidity of the tip increases.

[0020] According to a second aspect of the present invention, there isprovided a method for manufacturing a cutting tip, the method includinga preparation step of preparing a three-layer laminate wherein a secondlayer consisting of a hard sintered compact of CBN or diamond issandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide, a first cutting step of cutting out aprismatic blank of rectangular cross-section by cutting the first layer,the second layer and the third layer in order substantiallyperpendicularly to the upper face of the first layer, a second cuttingstep of cutting out a semi-completed tip including the second layer inthe middle thereof by cutting from one cut face of the prismatic blankto the other cut face, and a finishing step of obtaining a completed tipby forming on the semi-completed tip a rake face, a cutting edge and aflank.

[0021] Because the three-layer laminate is cut in strips andsemi-completed tips are cut out from the prismatic blanks obtained,semi-completed tips can be obtained in large numbers. Consequently yieldis good, and the manufacturing cost of the tip can be greatly reduced.

[0022] According to a third aspect of the present invention, there isprovided a cutting tool made up of a shank and a tip attached to theshank, wherein the tip is made from a three-layer laminate wherein asecond layer consisting of a hard sintered compact of CBN or diamond issandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide, and when the cutting tool is seen infront view, the second layer is a narrow band passing through the centerof rotation of the tool, a cutting edge is formed in this narrow band,and the second layer is reinforced on both sides by the first layer andthe third layer.

[0023] If an oil passage is formed in the shank and another oil passageis formed in the second layer of the tip and the two oil passagesconnect in a straight line, cutting oil can be injected through the oilpassages at the time of cutting. And these holes can be utilized aspositioning parts when the tip is attached to the shank.

[0024] According to a fourth aspect of the present invention, there isprovided a method for manufacturing a cutting tool made up of a shankand a tip attached to the shank, the method including a preparation stepof preparing a three-layer laminate wherein a second layer consisting ofa hard sintered compact of CBN or diamond is sandwiched by a first layerand a third layer consisting of a tool material such as cementedcarbide, a first cutting step of cutting out a prismatic blank ofrectangular cross-section by cutting the first layer, the second layerand the third layer in order substantially perpendicularly to the upperface of the first layer, a second cutting step of cutting out asemi-completed tip including the second layer in the middle thereof bycutting from one cut face of the prismatic blank to the other cut face,a joining step of joining the semi-completed tip to a separatelyprepared shank, and a finishing step of obtaining a completed tip byforming on the semi-completed tip a rake face, a cutting edge and aflank.

[0025] Thus, cutting tools are obtained by cutting a three-layerlaminate in strips, cutting out numerous semi-completed tips from theprismatic blanks obtained, attaching the semi-completed tips to shanks,and finishing the tips. Consequently, because yield is extremely goodand the manufacturing cost of the tips can be greatly reduced, themanufacturing cost of the cutting tool can be reduced.

[0026] Preferably, oil passages are made in advance in the shank and inthe semi-completed tip and in the joining step the semi-completed tip ispositioned on the shank by a pin being passed through the two holes andjoining of the semi-completed tip and the shank is carried out in thisstate. Because the semi-completed tip can be positioned with respect tothe shank by a pin being passed through the two oil passages like this,a cutting tool having good dimensional accuracy can be manufacturedeasily.

[0027] According to a fifth aspect of the present invention, there isprovided a method for manufacturing a cutting tip, the method includinga preparation step of preparing a three-layer laminate wherein a secondlayer consisting of a hard sintered compact of CBN or diamond issandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide, a first cutting step of cutting out aprismatic blank of rectangular cross-section by cutting the first layer,the second layer and the third layer in order substantiallyperpendicularly from the upper face of the first layer, a second cuttingstep of cutting out a semi-completed tip including the second layer inthe middle thereof by cutting the prismatic blank on a cutting planeorthogonal to or inclined at a predetermined angle to the cut face ofthe first cutting step, and a finishing step of obtaining a completedtip by forming on the semi-completed tip a rake face, a cutting edge anda flank.

[0028] According to a sixth aspect of the present invention, there isprovided a method for manufacturing a cutting tool made up of a shankand a tip attached to the shank, the method including a preparation stepof preparing a three-layer laminate wherein a second layer consisting ofa hard sintered compact of CBN or diamond is sandwiched by a first layerand a third layer consisting of a tool material such as cementedcarbide, a first cutting step of cutting out a prismatic blank ofrectangular cross-section by cutting the first layer, the second layerand the third layer in order substantially perpendicularly to the upperface of the first layer, a second cutting step of cutting out asemi-completed tip including the second layer in the middle thereof bycutting the prismatic blank on a cutting plane orthogonal to or inclinedat a predetermined angle to a cut face of the first cutting step, ajoining step of joining the semi-completed tip to a separately preparedshank, and a finishing step of obtaining a completed tip by forming onthe semi-completed tip a rake face, a cutting edge and a flank.

[0029] According to a seventh aspect of the present invention, there isprovided a thread-cutting tool made up of a shank and a tip attached tothe shank, wherein the tip is made up of a three-layer laminate in whicha second layer consisting of a hard sintered compact of CBN or diamondis sandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide, and, when the thread-cutting tool isseen in front view, the second layer is a thin band passing through thecenter of rotation of the tool, an end cutting edge and a thread-cuttingedge are formed in this narrow band, and the second layer is reinforcedon both sides by the first layer and the third layer.

[0030] A prepared hole is made with the end cutting edge of thethread-cutting tool, and thread-cutting is carried out with thethread-cutting edge. Because chamfering is also possible with thethread-cutting edge and the end cutting edge, the thread-cutting processcan be carried out with a single tool. This single thread-cutting toolis obtained by cutting a three-layer laminate along parallel lines,cutting out numerous semi-completed tips from the prismatic blanks thusobtained, attaching these semi-completed tips to shanks, and finishingthe tips. Accordingly, yield is good and the manufacturing cost of thethread-cutting tool can be reduced.

[0031] A flat drag of smaller diameter than the thread-cutting edge andlarger diameter than the shank is formed on the side of the tip attachedto the shank, and the end cutting edge, the thread-cutting edge and theflat drag are formed in this order in the second layer. While a threadis being cut with the thread-cutting edge, the rake simultaneously cutsflat the crests of the thread ridges, and the bottom of the threadedhole is finished by the end cutting edge. The reason for cutting thecrests of the thread ridges is to prevent the shank from making contactwith the thread ridges.

[0032] An oil passage is formed in the shank and an oil passage isformed in the second layer of the tip, and the two oil passages connectin a straight line. When thread-cutting is carried out, cutting oil isinjected through the oil passages.

[0033] According to an eighth aspect of the present invention, there isprovided a method for cutting a thread using a thread-cutting tool, themethod including a prepared hole machining step of making a preparedhole of substantially the same diameter as the external diameter of thethread-cutting tool by passing the thread-cutting tool into a cored holewhile rotating it about a threaded hole axis, an offsetting step ofoffsetting the axis of the thread-cutting tool from the threaded holeaxis by a predetermined distance after the end of the thread-cuttingtool reaches the bottom of the prepared hole and starting thread-cuttingin the prepared hole with a thread-cutting edge formed on the tool, anda thread-cutting step of cutting a thread with the thread-cutting edgeby rotating the axis of the thread-cutting tool about the threaded holeaxis while gradually withdrawing the thread-cutting tool incorrespondence with the lead of the thread.

[0034] Because the bottom of the threaded hole is finished at the sametime as the prepared hole is made with an end cutting edge, the depth ofthe prepared hole and the thread depth become essentially the same, andit is possible to dispense with an unthreaded part.

[0035] According to a ninth aspect of the present invention, there isprovided a method for cutting a thread using a thread-cutting tool, themethod including a thread-cutting step of cutting a thread in a coredhole with a thread-cutting edge by offsetting the axis of thethread-cutting tool from the threaded hole axis by a predetermineddistance and then turning the thread-cutting tool about the threadedhole axis and rotating the thread-cutting tool and advancing thethread-cutting tool in correspondence with the lead of the thread, athread ridge dragging step of dragging with a flat drag formed on thethread-cutting tool the crests of the ridges of the thread cut out withthe thread-cutting edge, and a bottom finishing step of finishing withan end cutting edge formed on the thread-cutting tool the bottom of thethreaded hole.

[0036] By this means it is possible to carry out thread-cutting with anadvancing movement of a thread-cutting tool without making a preparedhole in a cored hole. And because after the thread-cutting it is onlynecessary to remove the tool, the time required for thread-cutting canbe shortened.

[0037] According to a tenth aspect of the present invention, there isprovided a thread-cutting tool made up of a shank and a tip attached tothe shank, wherein the tip is made up of a three-layer laminate in whicha second layer consisting of a hard sintered compact of CBN or diamondis sandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide, and, when the thread-cutting tool isseen in front view, the second layer is a thin band passing through thecenter of rotation of the tool, a drill edge and a thread-cutting edgeare formed in this narrow band, and the second layer is reinforced onboth sides by the first layer and the third layer.

[0038] A prepared hole is made with the drill edge of the thread-cuttingtool, thread-cutting is carried out with the thread-cutting edge of thethread-cutting tool, and because chamfering is also possible with thedrill edge, the thread-cutting process can be carried out with a singletool.

[0039] At least two oil passages are provided in the shank, and to facethese oil passages at least one oil passage is provided in each of thefirst layer and the third layer of the tip so that the oil passages inthe shank and the oil passages in the tip are connected. When aplurality of oil passages are provided in this way, more cutting oil canbe injected, and thread-cutting can be carried out smoothly.

[0040] According to an eleventh aspect of the present invention, thereis provided a thread-cutting method including a prepared hole machiningstep of making a prepared hole in a workpiece with a drill edge formedon a thread-cutting tool by rotating the thread-cutting tool about athreaded hole axis, an offsetting step of offsetting the axis of thethread-cutting tool from the threaded hole axis by a predetermineddistance after the thread-cutting tool reaches the bottom of theprepared hole and starting thread-cutting in the prepared hole with athread-cutting edge formed on the thread-cutting tool, and athread-cutting step of cutting a thread with the thread-cutting edge byrotating the axis of the thread-cutting tool about the threaded holeaxis while gradually withdrawing the thread-cutting tool incorrespondence with the lead of the thread.

[0041] According to a twelfth aspect of the present invention. there isprovided a method for cutting a thread using a thread-cutting tool,which method comprises the steps of: making a prepared through hole in aworkpiece with a drill edge formed on a thread-cutting tool by rotatingthe thread-cutting tool about a threaded hole axis; chamfering an outletof the prepared through hole with a back of the drill edge; offsetting acenter axis of the thread-cutting tool from a threaded hole axis by apredetermined distance for cutting a thread in the prepared through holewith a thread-cutting edge formed on the thread-cutting tool; andcutting a thread in the through hole with the thread-cutting edge bygradually pulling the thread-cutting tool out from the hole incorrespondence with a lead of the thread while rotating the axis of thethread-cutting tool about the threaded hole axis.

[0042] In this arrangement, the prepared through hole is first formed inthe workpiece by means of the drill edge. At this time, a burr isproduced at a peripheral edge of an outlet of the prepared through hole.Then, a chamfer is provided at the outlet by using the back of the drilledge (part of the thread-cutting edge). The burr is removed uponchamfering. Continuously, thread cutting is performed on the preparedthrough hole upwardly from the outlet. In this arrangement, only asingle tool is thus required to achieve the prepared through holemachining, chamfering and thread cutting.

[0043] According to a thirteenth aspect of the present invention, thereis provided a hole-finishing drill made up of a shank and a tip attachedto the shank, wherein the tip is made up of a three-layer laminatewherein a second layer consisting of a hard sintered compact of CBN ordiamond is sandwiched by a first layer and a third layer consisting of atool material such as cemented carbide; when the drill is seen in frontview, the second layer is a thin band passing through the center ofrotation of the tool, a cutting edge is formed in this narrow band, anda pair of lands are formed on the periphery of the tip; the cutting edgeis a stepped edge formed with a plurality of steps in the form of astairway radially outward from the drill center; and guide pads forpreventing run out of the tip are formed projecting on the lands of thedrill.

[0044] Because the cutting edge is stepped, chips are broken up finely.Consequently, a chip discharge groove formed in the drill can be madesmall, and the rigidity of the drill can be increased. Since the guidepads formed on lands of the drill make contact with the wall face of thehole, run out of the tip is prevented, the wall face is cut smoothly,and the dimensional accuracy of the hole increases. In this way, ahole-making process and a finishing process can be carried out with asingle drill.

[0045] According to a fourteenth aspect of the present invention, thereis provided a cutting tool made up of a shank and a tip attached to theshank, wherein the tip is made up of a three-layer laminate wherein asecond layer consisting of a hard sintered compact of CBN or diamond issandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide; when the cutting tool is seen infront view, the second layer is a thin band passing through the centerof rotation of the tool, and a cutting edge is formed in this narrowband; and an end cutting edge constructed in the form of a stairway tokeep chips small, a thread-cutting edge for cutting a thread, and a flatdrag the same diameter as the internal diameter of the female thread areformed in the cutting edge in this order from the end of the tip towardthe shank.

[0046] The tip has an end cutting edge having a plurality of steps, athread-cutting edge and a flat drag. A chamfering process is carried outin advance by the thread-cutting edge. A hole-preparing process iscarried out by the end cutting edge and the thread-cutting edge. Crestsof ridges of a female thread having a predetermined internal diameterare formed by the flat drag the same diameter as the internal diameterof the female thread. Because a chamfer formed at the opening of thethreaded hole is made in advance like this, when the thread-cuttingprocess ends there is no formation of a burr at the threaded holeopening, and means for removing a burr are not necessary. Thus in thisinvention, machining of a chamfer, hole-preparing, thread-cutting, andmachining of thread ridges can be carried out with a single cuttingtool. As a result, there is no need for tool changes, and cutting workcan be carried out continuously, without stopping.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] A number of presently preferred embodiments of the invention willnow be described in detail below, by way of example only, with referenceto the accompanying drawings, in which:

[0048]FIG. 1 is a sectional view of a three-layer laminate employed inthe invention;

[0049]FIG. 2 is a plan view of FIG. 1 illustrating a method for cuttingnumerous prismatic blanks in parallel from a three-layer laminate in themanufacture of a tip;

[0050]FIG. 3 is a perspective view of a prismatic blank cut in FIG. 2;

[0051]FIG. 4 is a view illustrating oil passages formed in the prismaticblank shown in FIG. 3;

[0052]FIG. 5 is a view illustrating how semi-completed tips are cut outfrom the prismatic blank shown in FIG. 4;

[0053]FIG. 6 is a perspective view illustrating how a semi-completed tipis attached to a shank;

[0054]FIG. 7 is a side view of a shank and a semi-completed tip joinedtogether;

[0055]FIG. 8 is a side view of a cutting tool pertaining to a firstpreferred embodiment of the invention with a semi-completed tip finishedas a tip;

[0056]FIG. 9 is a view in the direction of the arrow 9 in FIG. 8;

[0057]FIG. 10 is a view corresponding to FIG. 5 illustrating howsemi-completed tips are cut out from the prismatic blank shown in FIG. 4to manufacture a tip having a different shape;

[0058]FIG. 11 is a front view of a finished tip made from asemi-completed tip cut out as illustrated in FIG. 10;

[0059]FIG. 12 is an exploded perspective view showing how the same tipis attached to a shank;

[0060]FIGS. 13A through 13D are views illustrating the manufacture of atip for a lathe tool from a three-layer laminate;

[0061]FIG. 14 is a view illustrating how the tip shown in FIG. 13D isattached to a shank;

[0062]FIG. 15 is a view illustrating a shaft being cut with the lathetool;

[0063]FIG. 16 is a view illustrating how wide prismatic blanks are cutin parallel from a three-layer laminate in the manufacture of a cuttingtip of a second preferred embodiment and is a plan view corresponding toFIG. 2;

[0064]FIG. 17 is a perspective view of a prismatic blank cut out in FIG.16;

[0065]FIG. 18 is a perspective view of a semi-completed tip cut from theprismatic blank shown in FIG. 17;

[0066]FIG. 19 is a front view of a semi-completed tip seen in thedirection of the arrow 19 in FIG. 18;

[0067]FIG. 20 is a perspective view of the same tip attached to a shank;

[0068]FIG. 21 is a view in the direction of the arrow 21 in FIG. 20;

[0069]FIGS. 22A and 22B are views illustrating the manufacture of athread-cutting tool from the semi-completed tip shown in FIG. 18;

[0070]FIG. 23 is a view in the direction of the arrow 23 in FIG. 22B;

[0071]FIG. 24 is a plan view of the thread-cutting tool;

[0072]FIG. 25 is a perspective view illustrating a method for cutting aprismatic blank when manufacturing end mills from prismatic blanks cutout from the three-layer laminate shown in FIG. 16;

[0073]FIG. 26 is a view in the direction of the arrow 26 in FIG. 25;

[0074]FIG. 27 is a front view of an end mill;

[0075]FIG. 28 is a view in the direction of the arrow 28 in FIG. 27;

[0076]FIG. 29 is a perspective view illustrating a method for cutting aprismatic blank when manufacturing taps from prismatic blanks cut outfrom the three-layer laminate shown in FIG. 16;

[0077]FIG. 30 is a perspective view of a semi-completed tip cut from theprismatic blank shown in FIG. 29;

[0078]FIG. 31 is a view in the direction of the arrow 31 in FIG. 30;

[0079]FIG. 32 is a front view of a tap;

[0080]FIG. 33 is a view in the direction of the arrow 33 in FIG. 32;

[0081]FIGS. 34A through 34F are views illustrating steps in a processfor cutting a thread using the cutting tool shown in FIG. 8 and FIG. 9as a first preferred embodiment;

[0082]FIG. 35 illustrates a comparison between a threaded hole inrelated art and a threaded hole formed by a threading method accordingto the invention;

[0083]FIG. 36 illustrates another preferred embodiment of athread-cutting tool having a flat drag, (b) being a front view on theline b-b in the side view of (a) and (c) being a sectional view on theline c-c in the side view of (a);

[0084]FIGS. 37A through 37F are views illustrating steps in a processfor cutting a thread using the thread-cutting tool shown in FIG. 36,FIG. 37D being an enlarged view of the part A in FIG. 37C;

[0085]FIG. 38 is a front view corresponding to FIG. 4 of a prismaticblank, showing an example of oil passages being provided in a firstlayer and a third layer;

[0086]FIG. 39 is an enlarged partial front view of the prismatic blankshown in FIG. 38, illustrating how semi-completed tips are cut out fromthe prismatic blank;

[0087]FIG. 40 is an exploded perspective view showing how asemi-completed tip cut out as shown in FIG. 39 is attached to a shank;

[0088]FIG. 41 is a side view of the shank and the semi-completed tipjoined together;

[0089]FIG. 42 is a side view of a thread-cutting tool made by forming athread-cutting edge on the semi-completed tip;

[0090]FIG. 43 is a front view of the thread-cutting tool seen in thedirection of the arrow 43 in FIG. 42;

[0091]FIGS. 44A through 44F are views illustrating steps in a processfor cutting a thread using the thread-cutting tool shown in FIG. 42 andFIG. 43;

[0092]FIG. 45 illustrates a comparison between a threaded hole inrelated art and a threaded hole formed by a threading method accordingto the invention;

[0093]FIGS. 46A through 46F are views illustrating steps in a processfor forming a threaded through hole using the cutting tool shown inFIGS. 42 and 43;

[0094]FIG. 47 is a side view of a hole-finishing drill according to theinvention;

[0095]FIG. 48 is an enlarged view of the part 48 in FIG. 47;

[0096]FIG. 49 is a front view of the drill as seen in the direction ofthe arrow 49 in FIG. 48;

[0097]FIGS. 50A through 50C are views illustrating an action of thehole-finishing drill when cutting a hole;

[0098]FIG. 51 is a sectional view on the line 51-51 in FIG. 50C;

[0099]FIGS. 52A through 52D are graphs comparing results obtained withdifferent pad angles 45° and 90°;

[0100]FIG. 53 is a side view showing another preferred embodiment of acutting tool;

[0101]FIG. 54 is an enlarged view of the part 54 in FIG. 53;

[0102]FIG. 55 is a front view of the cutting tool as seen in thedirection of the arrow 55 in FIG. 54;

[0103]FIG. 56 is a perspective view showing details of the shape of thetip shown in FIG. 54;

[0104]FIGS. 57A through 57D are views illustrating steps in a processfor cutting a thread in a cored hole in a casting by means of thecutting tool shown in FIGS. 54 through 56;

[0105]FIGS. 58A through 58D are views illustrating steps in a processfor cutting a thread in which dose not have a cored hole in a casting bymeans of the cutting tool shown in FIGS. 54 through 56; and

[0106]FIGS. 59A through 59C are views illustrating steps in a typicalthread-cutting process in related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0107] In FIG. 1, a three-layer laminate 10 has a layered structurewherein a second layer 11 is made from a hard sintered compact of cubicboron nitride (CBN) or diamond and this second layer 11 is sandwiched bya first layer 12 and a third layer 13 made from a tool material such ascemented carbide. For example the thickness of the second layer 11 is 1mm, the thicknesses of the first layer 12 and the third layer 13 areeach 5 mm, and the thickness of the three-layer laminate 10 as a wholeis 11 mm.

[0108] CBN is a man-made abrasive grain which, along with diamond, isused widely in cutting tips. CBN is suited to the cutting of ferrousworkpieces, and diamond is suited to the cutting of nonferrousworkplaces.

[0109] As an example of a method for manufacturing the three-layerlaminate 10, first the second layer 11 is manufactured by sintering 4 μmto 16 μm diamond particles or CBN particles under a constant pressure byHot Isostatic Pressing (HIP), and then the three-layer laminate 10 isobtained by laying tungsten-carbide (WC) powder on both sides of thissecond layer 11 and sintering under pressure by HIP.

[0110] In this invention, HIP may be replaced with HP (Hot Pressing) orCold Isostatic Pressing (CIP), and indeed any known sintering method maybe used.

[0111]FIG. 2 is a plan view of the three-layer laminate shown in FIG. 1.In FIG. 2, by cutting the three-layer laminate 10 along multiple cuttinglines 15 and essentially perpendicular to the top face of the firstlayer 12, numerous prismatic blanks 20 are cut out. As is clear from thefigure, the prismatic blanks 20 can be cut out with extremely good yield(for example 90% yield).

[0112] An enlarged view of one of the prismatic blanks 20 thus cut outis shown in FIG. 3. If the width of cutting out of the prismatic blank20 is written a and the thickness of the three-layer laminate is writtenb, then the prismatic blank 20 is a long-by-narrow member with arectangular cross-section a×b and is a laminate of the first layer 12,the second layer 11 and the third layer 13. Of the mutually opposing cutfaces of this prismatic blank 20, the right face in FIG. 3 will becalled the cut face 21 and the left face will be called the cut face 22.For convenience, the prismatic blank 20 is laid on its side in thedirection of the arrow c in FIG. 3 so that the cut face 21 faces upward.

[0113]FIG. 4 shows a prismatic blank 20 having oil passages fortransporting cutting oil. A plurality of these oil passages 24 areprovided with a predetermined pitch in the second layer 11 of theprismatic blank 20. Specifically, these holes are made by electricdischarge machining using a pipe electrode from the cut face 21 to thecut face 22 (see FIG. 3), that is, from the front side of the drawingtoward the rear side.

[0114]FIG. 5 shows how semi-completed tips are cut out from thisprismatic blank 20. That is, numerous semi-completed tips 30 are eachcut out by the prismatic blank 20 being cut from the cut face 21 to thecut face 22 (see FIG. 3), or from the front side of the drawing to therear side, so as to include one of the oil passages 24. Specifically,they are cut out by wire cut electric discharge machining.

[0115]FIG. 6 is a perspective view of a semi-completed tip thus cut outand a shank. The semi-completed tip 30 is fixed to the end of a shank 50made by forming an oil passage 51 in a round bar of tool steel andfinishing the round bar to a predetermined diameter. Preferably a pin 54is passed through the oil passage 24 and the oil passage 51 to raise theaccuracy with which the semi-completed tip 30 is positioned. Thesemi-completed tip 30 is joined to the end of the shank 50 by beingbrazed to the shank 50 with brazing filler metal 52, whereby a combinedshank and semi-completed tip is manufactured, as shown in FIG. 7.

[0116] As shown in FIG. 8 and FIG. 9, a tip 40 is finished by forming onthe semi-completed tip 30 rake faces (a front rake face 41 a and a siderake face 41 b), cutting edges (a front cutting edge 42 a and a sidecutting edge 42 b), flanks (a front flank 43 a and a side flank 43 b),and a tip oil groove 44.

[0117] The rake face, the cutting edge and the flank are also formedbelow the tip oil groove 44 in the figure.

[0118] As a result, as shown in FIG. 9, when the cutting tool 60 is seenfrom the front, the second layer 11 is a narrow band passing through thecenter of rotation of the tool, and the cutting edges 42 a, 42 b areformed in this narrow band. The second layer 11 is reinforced on bothsides by the first layer 12 and the third layer 13. Also, by the tip oilgroove 44 being so provided as to include the oil passage 24, an ampleflow of cutting oil can be supplied to the part of the workpiece beingcut.

[0119] As will be clear from the foregoing description, a method formanufacturing a cutting tool according to this first preferredembodiment includes a first cutting step of cutting out the prismaticblank 20 of rectangular cross-section shown in FIG. 3 by cutting thethree-layer laminate 10 shown in FIG. 1 from the top face of the firstlayer 12 substantially perpendicularly in the order of the first layer12, the second layer 11 and the third layer 13, a second cutting step ofcutting out multiple semi-completed tips 30 (see FIG. 5) each includingthe second layer 11 in the middle thereof by cutting from one cut face21 to the other cut face 22 of the prismatic blank 20, a joining step ofjoining each of these semi-completed tips 30 to a separately preparedshank 50, and a finishing step of obtaining a completed tip 40 byforming on the semi-completed tip 30 rake faces 41 a, 41 b, cuttingedges 42 a, 42 b and flanks 43 a, 43 b.

[0120] By employing this manufacturing method, because product yield isextremely high, as is clear from FIG. 2 and FIG. 5, tips can bemanufactured in large quantities inexpensively. And as a result, thecutting tool can also be manufactured at a low cost.

[0121]FIG. 10 through FIG. 12 illustrate the manufacture of a tip havinga different shape from the tip shown in the first preferred embodiment.

[0122]FIG. 10 shows how semi-completed tips 30 having a different shapefrom the semi-completed tips 30 illustrated in FIG. 5 are cut out fromthe prismatic blank 20 in the same way. Specifically, numeroussemi-completed tips 30 are cut out by cutting from one cut face to theother cut face of the prismatic blank 20 having the plurality of oilpassages 24, in other words from the front side of the drawing to therear side, so as to include one of the oil passages 24 each time. Thiscutting is carried out by wire cut electric discharge machining in thesame way as in the case of FIG. 5.

[0123] By finishing each of the semi-completed tips 30 thus cut out asshown in FIG. 11, and forming thereon rake faces 41, 41, cutting edges42, 42 and flanks 43, 43, a completed tip 40 is manufactured accordingto explanation in FIG. 10. And in this case also, as is clear from FIG.11, the cutting edges 42, 42 are formed on the second layer 11, thehardest part of the tip.

[0124] This cutting tip 40 can serve in various uses, but as a typicalusage example, as shown in FIG. 12, the tip 40 is fixed to the end of ashank 50 made by forming an oil passage 51 in a round bar of tool steel,finishing the round bar to a predetermined diameter, and forming grooves53, 53 symmetrically in the bar in its length direction. Preferably apin 54 is passed through the oil passages 24, 51 to raise the accuracywith which the tip 40 is positioned. The tip 40 is brazed to the end ofthe shank 50 with brazing filler metal 52, and a cutting tool isobtained.

[0125] Thus a method for manufacturing a cutting tip according to thisfirst preferred embodiment includes a first cutting step of cutting outthe prismatic blank 20 of rectangular cross-section shown in FIG. 3 bycutting the three-layer laminate 10 shown in FIG. 1 from the top face ofthe first layer 12 substantially perpendicularly in the order of thefirst layer 12, the second layer 11 and the third layer 13, a secondcutting step of cutting out a semi-completed tip 30 (see FIG. 10)including the second layer 11 in the middle thereof by cutting from onecut face 21 to the other cut face 22 of the prismatic blank 20, and afinishing step of obtaining a completed tip 40 by forming on thesemi-completed tip 30 the rake faces 41, the cutting edges 42, and theflanks 43 shown in FIG. 11.

[0126]FIGS. 13A through 13D are views illustrating the manufacture of alathe tool.

[0127] In FIG. 13A, a disc-shaped three-layer laminate 10 is cut instrips along the dotted lines.

[0128]FIG. 13B shows one of the prismatic blanks 20 obtained. Forconvenience, this prismatic blank 20 is laid on its side in thedirection of the arrow d.

[0129] In FIG. 13C, multiple semi-completed tips 30 each including thesecond layer 11 in the middle thereof are cut out by cutting from onecut face 21 to the other cut face 22 by wire cut electric dischargemachining.

[0130] In FIG. 13D, finishing machining is carried out on thesemi-completed tips 30 to obtain tips 40.

[0131] Next, as shown in FIG. 14, an M-shaped groove 56 is cut in theend of a plate shank 50, the tip 40 is fitted in this M-shaped groove56, and the two are joined together by brazing to obtain a lathe tool.

[0132]FIG. 15 shows a usage example wherein a shaft 71 is being cut witha lathe tool 70 consisting of the-cutting tool obtained in FIG. 14.

[0133] Although in FIG. 3 and FIG. 13B the prismatic blank 20 was laidon its side, alternatively the prismatic blank 20 may be cut sidewaysfrom one cut face 21 to the other cut face 22 without being laid on itsside; that is, it may be cut in any machining attitude.

[0134]FIGS. 16 through 21 illustrate the manufacture of a drillconstituting a cutting tip pertaining to a second preferred embodiment.FIG. 16 corresponds to FIG. 2: multiple prismatic blanks 120 wider thanthe prismatic blanks 20 of FIG. 2 are cut out in parallel from athree-layer laminate 110 along cut lines 115. As in the example shown inFIG. 2, in this preferred embodiment also the prismatic blanks 120 canbe cut with good yield (for example 90%).

[0135]FIG. 17 shows a prismatic blank 120 thus cut out. The prismaticblank 120 is a long-by-narrow member with a square cross-section a×a ofwidth a and height a and is a laminate of the first layer 12, the secondlayer 11 and the third layer 13. Numerous semi-completed tips 130 areobtained by cutting the prismatic blank 120 on a cutting plane 122 bentin plan view in the shape of a dog leg so as to be inclined at apredetermined angle to a cut face 121 formed in FIG. 16.

[0136] In FIG. 18, a semi-completed tip 130 thus cut is a piece whosefront face 131 is pointed like an arrow outwardly and whose rear face132 is pointed like an arrow inwardly. This cutting is carried out forexample by wire cut electric discharge machining.

[0137] Next, a completed tip is manufactured from the semi-completed tip130 shown in FIG. 18. In FIG. 19, the semi-completed tip 130 is cut byelectric discharge machining along cut lines 133, from the front face ofthe figure toward the rear face. The cut lines 133 leave the secondlayer 11 almost entirely intact but cut greatly into the first layer 12and the third layer 13. Cutting the semi-completed tip 130 like thisproduces a tip 140.

[0138]FIG. 20 shows the tip 140 of this preferred embodiment attached toa shank. The tip 140 formed in FIG. 19 can be applied to various uses,for example a drill, a thread-cutting tool, an end mill, or a tap.Taking the specific example of a drill, the tip 140 is joined by brazingfiller metal 152 to the end of a drill-shaped shank 151 and providedwith edges to form a drill 150. That is, as shown in FIG. 21, a drill150 is manufactured by forming a chisel edge 153, grooves 154, 154, rakefaces 155, 155, cutting edges 156, 156 and flanks 157, 157 on the drill150 shown in FIG. 20.

[0139] As is clear from the foregoing description, a method formanufacturing a cutting tip according to this second preferredembodiment, explained here using the example of a drill, includes afirst cutting step of cutting out the multiple prismatic blanks 120shown in FIG. 16 by cutting the three-layer laminate 10 shown in FIG. 1from the top face of the first layer 12 substantially perpendicularly inthe order of the first layer 12, the second layer 11 and the third layer13, a second cutting step of cutting out the semi-completed tips 130(see FIG. 17) including the second layer 11 in the middle thereof bycutting the prismatic blanks 120, and a finishing step of obtainingcompleted tips 140 by forming on each of the semi-completed tips 130 therake faces 155, the cutting edges 156 and the flanks 157 shown in FIG.21.

[0140] As a result of employing this manufacturing method, as is clearfrom FIG. 16 and FIG. 17, product yield is extremely high, and tips canbe manufactured in large quantities inexpensively.

[0141] Also, a method for manufacturing a cutting tool according to thissecond preferred embodiment, explained here using the example of adrill, includes a preparation step of preparing a three-layer laminate10 wherein a second layer 11 made of a hard sintered compact of CBN ordiamond is sandwiched by a first layer 12 and a third layer 13 made of atool material such as cemented carbide, a first cutting step of cuttingout prismatic blanks 120 of rectangular cross-section by cutting thethree-layer laminate 10 from the top face of the first layer 12substantially perpendicularly in the order of the first layer 12, thesecond layer 11 and the third layer 13, a second cutting step of cuttingout semi-completed tips 130 each including the second layer 11 in themiddle thereof by cutting the prismatic blanks 120 along cutting planes122 bent so that cutting is carried out at a predetermined angle to thecut face 121 formed in the first cutting step, a joining step of joiningthe semi-completed tips 130 to separately prepared shanks 151, and afinishing step of obtaining completed tips 140 by forming on each of thesemi-completed tips 130 the rake faces 155, the cutting edges 156 andthe flanks 157. By this manufacturing method a cutting tool such as thedrill 150 can be manufactured.

[0142] Next, a method for manufacturing a thread-cutting tool from thesemi-completed tip 130 shown in FIG. 18 will be described.

[0143] As shown in FIG. 22A, the semi-completed tip 130 shown in FIG. 18is cut along cut lines 134, 134 showing with a broken line. If thesemi-completed tip 130 is considered a primary semi-completed tip, thenas a result of this cutting a secondary semi-completed tip 135 isobtained as shown in FIG. 22B.

[0144] Then, as shown in FIG. 23 looking at the outwardly pointing face131, which is the front face of the secondary semi-completed tip 135shown in FIG. 22B, the secondary semi-completed tip 135 is cut along cutlines 136, from the front face toward the rear face of the drawing, byelectric discharge machining. The cut lines 136 leave the second layer11 almost entirely intact but cut greatly into the first layer 12 andthe third layer 13. The cut semi-completed tip 130 becomes a tip 140 ofa thread-cutting tool.

[0145]FIG. 24 shows a thread-cutting tool having the tip 140. Thisthread-cutting tool 160 is made by joining the tip 140 to the end of ashank 161 with brazing filler metal 162 and forming on it an end cuttingedge 163 and thread-cutting edges 164, 164.

[0146] Next, the manufacture of an end mill will be described, withreference to FIGS. 25 through 28.

[0147] In FIG. 25, the prismatic blank 120 is a long-by-narrow memberwith a square cross-section a×a, and is a laminate of the first layer12, the second layer 11 and the third layer 13. By cutting thisprismatic blank 120 on multiple cutting planes 123 orthogonal to the cutface 121 shown in FIG. 25, multiple semi-completed tips 130 shaped likedice are obtained.

[0148] In FIG. 26, a semi-completed tip 130 is cut by electric dischargemachining along cut lines 137, from the front side toward the rear sideof the drawing. The cut lines 137 leave the second layer 11 almostentirely intact but cut greatly into the first layer 12 and the thirdlayer 13. The semi-completed tip 130 thus cut becomes a tip 140.

[0149] Then, the tip 140 obtained by cutting in FIG. 26 is joined withbrazing filler metal 172 to a shank 171 shown in FIG. 27 and an end mill170 is obtained. The end mill 170 is a cutting tool for cutting aworkpiece 174 by rotating while being fed as shown by the large arrow.

[0150] As shown in FIG. 28, rake faces 175, 175, cutting edges 176, 176and flanks 177, 177 are formed on the end of the end mill 170.

[0151] Discussion will be made next as to the manufacture of tap withreference to FIGS. 29 through 33.

[0152] In FIG. 29, the prismatic blank 120 is a long-by-narrow emberwith a square cross-section a×a, and is a laminate of the first layer12, the second layer 11 and the third layer 13. By cutting thisprismatic blank 120 on multiple cutting planes 123 orthogonal to the cutface 121 shown in FIG. 29, multiple semi-completed tips 130 shaped likedice are obtained.

[0153] Then, on both side faces of the primary semi-completed tip 130obtained in FIG. 29, sawtooth parts 181, 181 are formed as shown in FIG.30, and a secondary semi-completed tip 135 is obtained.

[0154] In FIG. 31, the secondary semi-completed tip 135 is cut byelectric discharge machining along cut lines 138, from the front sidetoward the rear side of the drawing. The cut lines 138 leave the secondlayer 11 almost entirely intact but cut greatly into the first layer 12and the third layer 13. The secondary semi-completed tip 135 thus cutbecomes a tip 140.

[0155] The tip 140 obtained in FIG. 31 is joined to a shank 182 withbrazing filler metal 183 and a tap 180 is obtained, as shown in FIG. 32.

[0156] As shown in FIG. 33, rake faces 185, 185, cutting edges 186, 186and flanks 187, 187 are formed on the end of the tap 180.

[0157] A cutting tip pertaining to the second preferred embodimentdescribed above can be used as a tip of a cutting tool such as a millingcutter or a lathe tool as well as for a drill, a thread-cutting tool, anend mill or a tap as described in this preferred embodiment. Thus, theform of use of a cutting tip manufactured in accordance with theinvention can be determined freely.

[0158] Next, a method for using the cutting tool 60 shown in FIG. 8 andFIG. 9 as a thread-cutting tool and cutting a thread using thisthread-cutting tool 60 will be described.

[0159]FIGS. 34A through 34F illustrate steps in a thread-cutting processusing the thread-cutting tool 60.

[0160]FIG. 34A: The rotating thread-cutting tool 60 is brought to face acored hole 62 and machining of a prepared hole is started.

[0161]FIG. 34B: The internal diameter of the prepared hole 63 isapproximately the same as the external diameter of the thread-cuttingtool 60. Advancing (lowering) of the thread-cutting tool 60 is stoppedwhen it reaches a predetermined depth.

[0162]FIG. 34C: The axis 66 of the thread-cutting tool is offset by adistance δ from the threaded hole axis 64. Because the thread-cuttingtool 60 is rotating, the wall forming the prepared hole 63 can be cutinto easily.

[0163]FIG. 34D: A thread 68 is cut with the thread cutting edge 42 b bythe thread-cutting tool 60 being gradually withdrawn in correspondencewith the lead L of the thread 68 while the axis 66 of the thread-cuttingtool is rotated about the threaded hole axis 64.

[0164]FIG. 34B: When the thread cutting edge 42 b of the thread-cuttingtool 60 reaches the entrance of the thread 68, the axis 66 of thethread-cutting tool is offset further from the threaded hole axis 64 toform a chamfer 69.

[0165]FIG. 34F: The thread-cutting tool 60 is removed and thread-cuttingis ended.

[0166]FIG. 35B shows a female thread formed by this method and FIG. 35Aa comparison example.

[0167] (a) of FIG. 35 is copied from FIG. 58C and shows a thread 515manufactured by a thread-cutting method of related art, including anunthreaded part of depth (D1-D2).

[0168] (b) of FIG. 35 is copied from FIG. 34F and shows a thread 68manufactured by the thread-cutting method of this preferred embodiment,in which there is no unthreaded part of depth (D1-D2).

[0169] Accordingly, with the method of this preferred embodiment, thecasting can be made thinner. That is, because it is not necessary toincrease its thickness to form the thread, the casting can be madelighter.

[0170] (a) through (c) of FIG. 36 illustrate another preferredembodiment of a thread-cutting tool, having a different structure fromthe cutting tool 60. Parts in these figures, which are the same as partsin FIG. 8 and FIG. 9, have been given the same reference numerals.

[0171] As shown in (a) of FIG. 36, a thread-cutting tool 60A has a tip40 attached by brazing filler metal 52 to a shank 50. On the shank 50side of the tip 40, a flat drag 75 of smaller diameter than thethread-cutting edge and 42 b and larger diameter than the shank 50 isformed.

[0172] As shown in (b) of FIG. 36, when the tool is seen from the front,the second layer 11 is a narrow band passing through the center ofrotation of the tool. End cutting edges 42 a, 42 a and thread-cuttingedges 42 b, 42 b are formed in this narrow band. The second layer 11 isreinforced on both sides by the first layer 12 and the third layer 13.

[0173] As shown in (c) of FIG. 36, flat drags 75, 75 are formed on thesecond layer 11. The second layer 11 is reinforced on both sides by thefirst layer 12 and the third layer 13.

[0174] Next, a process for cutting a thread using the thread-cuttingtool 60A shown in (a) of FIG. 36 will be described with reference toFIGS. 37A through 37F.

[0175]FIG. 37A: First, the thread-cutting tool 60A, rotating at apredetermined speed, is brought to face a cored hole 62.

[0176]FIG. 37B: Then, by cutting the entrance of the cored hole 62 withthe thread-cutting tool 60A, a chamfer 69 is formed.

[0177]FIG. 37C: The tool axis 66 is offset from the threaded hole axis64 by a predetermined distance δ. Then, the thread-cutting tool 60A isturned about the threaded hole axis 64 while being rotated, and also thethread-cutting tool 60A is advanced (moved downward in the drawing) incorrespondence with the lead of the thread 68. In this way, a thread 68is formed with the cutting edge 42 b in the wall of the cored hole 62.

[0178]FIG. 37D: (This is a detail view of the part A in FIG. 37C.)Immediately after the thread is cut with the thread cutting edge 42 b,and specifically {fraction (1/2)} of the lead of the thread later, thatis, ½ of a rotation of the tool later, the crest 68 a of the ridge ofthe thread is cut by the flat drag 75. As a result, a gap β ismaintained between the crest 68 a of the ridge of the thread and theshank 50. If this cut were not carried out, the shank 50 would makecontact with the crest of the ridge and both the thread and the shank 50would be damaged. When, however, as in this example, the crest 68 a ofthe ridge of the thread is dragged (made flat) immediately after thethread is cut, this kind of trouble does not arise.

[0179] In FIG. 37E, after the thread 68 is cut to a predetermined depth,the bottom 73 of the threaded hole is finished and flattened by the endcutting edge 42 a.

[0180] In FIG. 37F, the thread-cutting tool 60A is removed. Because nospecial movement is needed for this removal step, that is, whenwithdrawing the thread-cutting tool 60A, the thread-cutting tool 60A canbe removed swiftly.

[0181] Whereas in the preferred embodiment illustrated in FIGS. 34Athrough 34F a prepared hole was made in the advancing movement of thethread-cutting tool and thread-cutting was carried out the a withdrawingmovement, in the preferred embodiment illustrated in FIGS. 37A through37F thread-cutting is performed in the advancing movement without aprepared hole being made. And since withdrawal consists of just removingthread-cutting tool, rapid withdrawal is possible, and the time requiredfor thread-cutting can be greatly shortened.

[0182] The thread-cutting tool of this preferred embodiment is ideal forthe thread-cutting of aluminum alloy castings. And since aluminum alloyis soft and its cutting resistance is relatively low, a satisfactorylife of the thread-cutting tool can be expected. However, thread-cuttingin various other castings, such as gray cast iron (PC), ductile castiron (FCD), and stainless steel castings (SCS), or in resin or copper,can also be carried out with the thread-cutting tool of this preferredembodiment.

[0183] Next, a thread-cutting tool illustrating another preferredembodiment will be described.

[0184]FIG. 38 is a view corresponding to FIG. 4, showing a prismaticblank of the first preferred embodiment. Parts the same as in the firstpreferred embodiment have been given the same reference numerals.

[0185] In this preferred embodiment, a plurality of oil passages 24 aremade at a predetermined pitch in each of the first layer 12 and thethird layer 13 of the prismatic blank 20. The oil passages 24 in thefirst layer 12 and the oil passages 24 in the third layer 13 aredisposed alternately. Specifically, these holes are made by electricdischarge machining using a pipe electrode from the cut face 21 to thecut face 22, or from the front side of the drawing toward the rear.

[0186] In FIG. 39, numerous semi-completed tips 30 are cut out by theprismatic blank 20 being cut from one cut face 21 to the other cut face,or from the front of the drawing toward the rear, so as to include oneoil passage 24 disposed in the first layer 12 and one oil passage 24disposed in the third layer 13. Specifically, they are cut out by wirecut electric discharge machining.

[0187] In FIG. 40, two oil passages 51, 51 are made in the lengthdirection of a round bar of tool steel to serve as a shank 50 so as tocorrespond to the oil passages 24, 24 provided in the first layer 12 andthe third layer 13, and the round bar is finished to a predetermineddiameter and has grooves 53, 53 formed in two opposite sides thereof. Asemi-completed tip 30 is brought to the end of the shank 50, preferablythe positioning accuracy of the semi-completed tip 30 is raised by pins54, 54 being passed through the oil passages 24, 24, 51, 51, and thesemi-completed tip 30 is brazed to the shank 50 with brazing fillermetal 52. The semi-completed tip 30 joined to the end of the shank 50with brazing filler metal 52 is shown in FIG. 41.

[0188] In FIG. 42 and FIG. 43, the tip 40 is finished by rake faces (afront rake face 41 a and a side rake face 41 b), cutting edges (a drilledge 46 a and a thread-cutting edge 46 b) and flanks (a front flank 43 aand a side flank 43 b) being formed on the semi-completed tip 30.

[0189] As shown in FIG. 43, when this, thread-cutting tool 60B is seenfrom the front, the second layer 11 is a narrow band passing through thecenter of rotation of the tool, the drill edge 46 a and thethread-cutting edge 46 b are formed in this narrow band, and the secondlayer 11 is reinforced on both sides by the first layer 12 and the thirdlayer 13.

[0190] Because the oil passages 24, 24 open at the front flanks 43 a, 43a, an ample flow of cutting oil can be supplied to the part of theworkpiece being cut.

[0191] Next, a thread-cutting process using the thread-cutting tool 60Bof this preferred embodiment shown in FIG. 42 and FIG. 43 will bedescribed on the basis of FIGS. 44A through 44F.

[0192]FIG. 44A: The rotating thread-cutting tool 60B is brought to facea casting 61, and machining of a prepared hole is started. A preparedhole can be machined with the drill edge 46 a.

[0193]FIG. 44B: The internal diameter of the prepared hole 63 isapproximately the same as the external diameter of the thread-cuttingtool 60B. Advancing (descending) of the thread-cutting tool 60B isstopped when it reaches a predetermined depth.

[0194]FIG. 44C: The axis 66 of the thread-cutting tool is offset fromthe threaded hole axis 64 by a distance δ. Because the thread-cuttingtool 60B is rotating, it can easily cut into the wall of the preparedhole 63.

[0195]FIG. 44D: A thread 68 is cut with the thread cutting edge 42 b bythe thread-cutting tool 60B being gradually withdrawn in correspondencewith the lead L of the thread 68 while the axis 66 of the thread-cuttingtool 60B is rotated about the threaded hole axis 64.

[0196]FIG. 44E: When the thread-cutting edge 46 b of the thread-cuttingtool 60B reaches the entrance of the thread 68, the axis 66 of thethread-cutting tool is offset further from threaded hole axis 64 to forma chamfer 69.

[0197]FIG. 44F: The thread-cutting tool 60B is removed andthread-cutting is ended.

[0198] (a) and (b) of FIG. 45 show for comparison completed threads ofrelated art and this preferred embodiment.

[0199] (a) of FIG. 45 is a comparison example and is copied from FIG.58C and shows a thread 515 manufactured by a thread-cutting method ofrelated art, of depth D1.

[0200] (b) of FIG. 45 is copied from FIG. 44F and shows a thread 68manufactured by the thread-cutting method of the present preferredembodiment, of depth D3.

[0201] As is clear from these figures, the depth D3 is shallower thanthe depth D1, and thus with the present preferred embodiment it ispossible to make the depth of a threaded hole shallower by (D1-D3) thanin related art. And therefore, with the method of this preferredembodiment, it is possible to make the casting thinner. That is, becauseit is not necessary to increase its thickness to form the thread, thecasting can be made lighter.

[0202] Next, with reference to FIGS. 46A through 46C, discussion will bemade as to steps in a process for forming a threaded through hole in acasting as a workpiece. These figures includes top plan views at upperlevels thereof.

[0203]FIG. 46A: The thread-cutting tool 60B is brought into confrontingrelation to the casting 82 as the workpiece for starting the machiningof a prepared hole in the casting 82.

[0204]FIG. 46B: A prepared through hole 83 is formed in the casting 82by means of the drill edge 46 a, whereupon a burr 85 is produced at aperipheral edge of an outlet 84 of the prepared hole 83. The burr 85 isa small or thin piece of uncut remains. The prepared hole 83 has aninside diameter substantially equal to an outer diameter of thethread-cutting tool 60B.

[0205]FIG. 46C: The central axis 66 of the thread-cutting tool 60B iscaused to offset a distance δ1 from the threaded hole axis 64. This isfollowed by ascending the tool 60B to carry out chamfering with a back46 c (part of the thread-cutting edge 46 b) of the drill edge 46 a. Thatis, a chamfer 86 is provided at the outlet 84 of the prepared hole 83 bycausing the tool 60B to revolve about the threaded hole axis uponrotation of the tool 60B on its axis.

[0206] Since chamfering of the outlet 84 can thus be effectedcontinuously with the formation of the prepared hole 83, it becomesunnecessary to change the tool to another for chamfering.

[0207]FIG. 46D: An offset of the central axis 66 of the thread-cuttingtool 60B relative to the threaded hole axis 64 is changed to δ. In thisstate, threads are cut in the prepared hole 83 by means of thethread-cutting edge 46 b

[0208]FIG. 46E: The thread-cutting tool 60B is gradually pulled upwardlyin correspondence with the lead L of the threaded portion 68 whilerotating the thread-cutting tool 60B about the threaded hole axis 64,thereby forming the threaded portion 68 by means of the thread-cuttingedge 46 b.

[0209]FIG. 46F: Upon completion of the thread cutting, thethread-cutting tool 60B is caused to be largely offset from the threadedhole axis 64. Thereafter, a chamfer 69 is provided at an inlet of thethreaded hole. Finally, the thread-cutting tool 60B is removed tothereby complete the thread cutting.

[0210] As explained above, since the chamfer 86 is provided at theoutlet 84 of the prepared hole 83 by using the back 42 c (part of thethread-cutting edge 46 b) of the drill edge 46 a of the tool 60B, theoutlet 84 has no burrs formed thereat.

[0211] The thread-cutting tool of this preferred embodiment is ideal forthe thread-cutting of aluminum alloy castings. And since aluminum alloyis soft and its cutting resistance is relatively low, a satisfactorylife of the thread-cutting tool can be expected. However, thread-cuttingin various other castings, such as gray cast iron (FC), ductile castiron (FCD), and stainless steel castings (SCS), or in resin or copper,can also be carried out with the thread-cutting tool of this preferredembodiment.

[0212] And in this preferred embodiment, two oil passages were made,each extending from the shank to the tip. If a pair of oil passages aremade symmetrically about the center of rotation like this, therotational balance of the thread-cutting tool can be made good. However,there may alternatively be one or three or more oil passages, the numberof oil passages being freely determinable as long as they open at theflanks or the rake faces of the tip.

[0213]FIGS. 47 through 49 show a hole-finishing drill according to thispreferred embodiment. This hole-finishing drill 210 is a cutting toolmade by fixing a tip 213 to the end of a shank 211 with brazing fillermetal 212.

[0214] The shank 211 is a round bar of cemented carbide steel finishedto a predetermined diameter, provided with oil passages, and having itsend formed into a point. By forming the end into a point like this theaccuracy of positioning of the tip 213 can be raised and the strength ofits fixing increased.

[0215] As shown in FIG. 48 and FIG. 49 cutting edges 216, cutting parts217, lands 218 and guide pads 219 are formed on the tip 213.

[0216] The cutting edges 216 are edges for making a hole in a workpiece,and each is a stepped edge made by forming a first cutting edge 221 atthe center of the drill, forming a second cutting edge 222 radiallyoutward from the first cutting edge 221 with a step of a stairway formtherebetween, and forming a third cutting edge 223 radially outward fromthe second cutting edge 222 with a step of a stairway form therebetween.S denotes the step formed between the first cutting edge 221 and thesecond cutting edge 222 and the step formed between the second cuttingedge 222 and the third cutting edge 223. The reference numeral 225denotes rake faces of the cutting edges 216 and the cutting parts 217,the reference numeral 226 denotes flanks of the cutting edges 216, and θis the point angle of the cutting edges 216.

[0217] The cutting parts 217 are hole-finishing edges, and are formedprojecting from the periphery of the tip 213. The reference numeral 227denotes run offs of the cutting parts 217.

[0218] The guide pads 219 are for preventing run out of the tip 213 andare formed projecting from the lands 218 of the tip 213. By the guidepads 219 being formed near the cutting parts 217, they are provided witha length L. The reference numeral 228 denotes run offs of the guide pads219.

[0219] It is shown in FIG. 49 that the guide pads 219 are formed nearthe cutting parts 217, and specifically with the distance from thecutting parts 217 to the guide pads 219 as a pad angle β. The pad angleβ is preferably 45°.

[0220] The tip 213 has a structure wherein a second layer 11 issandwiched between a first layer 12 and a third layer 13. The secondlayer 11 is a narrow band passing through the center of rotation of thetool, the cutting edges 216 are formed in this narrow band, and thesecond layer 11 is reinforced on both sides by the first layer 12 andthe third layer 13. The reference numerals 235, 235 denote oil passagesand the reference numerals 236, 236 denote chip discharge grooves.

[0221] For the tip 213, as described above with reference to FIG. 1 andFIGS. 16 through 18, a semi-completed tip is obtained from a three-layerlaminate and then this semi-completed tip is brazed to the end of theshank 211 with the brazing filler metal 212. The cutting edges 216, theguide pads 219 and the chip discharge grooves 236 are then formed on thesemi-completed tip to complete the hole-finishing drill 210.

[0222] Next, action of this hole-finishing drill 210 will be describedwith reference to FIGS. 50A through 50C and FIG. 51.

[0223] In FIG. 50A, with the hole-finishing drill 210 being rotated andcutting oil 251 being supplied, the hole-finishing drill 210 is broughtto face a casting 252 and hole-making is started.

[0224] In FIG. 50B, the casting 252 is cut with the cutting edges 216and a hole 253 is formed in the casting 252. Because the cutting edges216 are each divided into a first cutting edge 221, a second cuttingedge 222 and a third cutting edge 223, the lengths of the individualedges are small, and consequently the chips 254 produced are small. Whenthe chips 254 are small, chips can be easily discharged even if thecross-sectional area of the chip discharge grooves 236 is small;consequently, the chip discharge grooves 236 can be made small to securerigidity of the hole-finishing drill 210. As a result, vibration of theshank 211 occurs less readily and the accuracy of the hole increases.

[0225] Because the material of the cutting edges 216 is a hard sinteredcompact of CBN or diamond, the cutting speed (speed of rotation) of thehole-finishing drill 210 can be increased and productivity can beimproved.

[0226] In FIG. 50C, following the cutting edges 216, the corners of thecutting parts 217, 217 cut the wall face 255 of the hole 253 slightly,and while the wall face 255 is pressed by the cutting parts 217, 217 theguide pads 219 make contact with the wall face 255, and run out isprevented. Consequently, the wall face 255 of the hole 253 is smooth,the surface roughness of the wall face 255 is low and the dimensionalaccuracy (dimensional tolerance) of the hole 253 improves.

[0227] Because the hole 253 is finished like this as it is made by thehole-finishing drill 210, a hole-making step and a hole-finishing stepcan be performed in one pass, and the machining time can be shortened.For example, when machining a hole of diameter 10 mm and depth 5 mmunder a given condition, a highly accurate hole can be obtained in amachining time of 0.2 seconds (total) in one pass, and the machiningtime can be cut by 50%.

[0228] Also, because the hole is finished with the one hole-finishingdrill 210 alone, it is not necessary to use both a drill for making thehole 253 and a reamer for finishing the hole 253, and tool-changing timeis thus reduced. For example, when machining a hole of diameter 10 mmand depth 5 mm under a given condition, since there is no tool-changingtime, a time of approximately 4 seconds can be saved.

[0229] In FIG. 51, by rotating the cutting parts 217, 217 and the guidepads 219, 219 clockwise in the hole 253 as shown by the central arrow,the wall face 255 of the hole 253 is finished to a high accuracy by thecutting parts 217, 217. That is, when a cutting force acts on thecutting parts 217, the guide pads 219, 219 projecting from the lands218, 218 make contact with the wall face 255 as shown by the straightarrows, and consequently the axis of the tip 213 does not vibratelargely and accuracy (surface roughness and dimensional tolerance) ofthe hole 253 is secured.

[0230] Also, if the guide pads 219 are formed in positions at a padangle β as shown in FIG. 49, the surface roughness of the wall face 255stabilizes and the dimensional accuracy (dimensional tolerance) of thehole 253 stabilizes. If the pad angle β is less than 45°, the guide pads219 are too close to the cutting parts 217 and the guide pads 219 tendnot to make contact with the wall face 255 and run out of the tip 213tends to occur. If the pad angle β is 45°, run out of the tip 213 can beprevented. If the pad angle β exceeds 45°, the guide pads 219 are toofar from the cutting parts 217 and even if the guide pads 219 makecontact with the wall face 255 they can less easily stop run out of thetip 213.

[0231] Next, an example of a test of the pad angle β will be described.

[0232]FIGS. 52A through 52D are graphs comparing the case of padangle=45° with the case of pad angle=90°. In FIG. 52A and FIG. 52B thehorizontal axis shows feed per revolution and the vertical axis showsvariation of tolerance of the hole. In FIG. 52C and FIG. 52D thehorizontal axis shows feed per revolution and the vertical axis showssurface roughness.

[0233]FIG. 52A: In the case of pad angle 45°, the variation of toleranceof the hole is substantially constant with respect to increasing feedrate, and thus the dimensions of the hole are stable.

[0234]FIG. 52B: In the case of pad angle 90°, the variation of toleranceof the hole decreases in proportion with increasing feed rate, and thusdispersion arises in the dimensions of the hole.

[0235]FIG. 52C: In the case of pad angle 45°, the surface roughness isconstant with respect to increasing feed rate, and thus the surfaceroughness is stable.

[0236]FIG. 52D: In the case of pad angle 90°, the surface roughness mayincrease with varying feed rate.

[0237] In this preferred embodiment, an example wherein three steps areformed in each of the cutting edges 216, as illustrated in FIG. 47, hasbeen shown; however, in the invention the number of steps is not limitedto three, and the number of steps may be varied in accordance with thesize of the hole diameter.

[0238] Also, although the number of guide pads 219 shown in FIG. 49 istwo, the number is not limited to two. And similarly, the number ofcutting edges 216 and the number of cutting parts 217 can be determinedfreely.

[0239] As described above, in this preferred embodiment, because eachcutting edge of a hole-finishing drill is made a stepped edge having aplurality of steps in the form of a stairway radially outward from thecenter of the drill, chips can be broken up finely. And because of this,the chip discharge grooves can be made small and the rigidity of thedrill can be increased.

[0240] Also, because guide pads for preventing run out of the tip areformed projecting from lands of the drill, the guide pads make contactwith the wall of the hole and the tip does not vibrate during cutting.Consequently, the: wall of the hole can be finished to a high accuracy.

[0241] Thus while making a hole it is possible to perform finishing ofthe hole as well.

[0242] Next, another preferred embodiment of a cutting tool forthread-cutting, shown in FIGS. 53 through 56, will be described. In FIG.53, a cutting tool 310 of this preferred embodiment is made up of a tip313 fixed to the end of a shank 311 with brazing filler metal 312. Theshank 311 is a round bar of tool steel finished to a predetermineddiameter, provided with oil passages, and having its end formed into apoint. By forming the end into a point like this the accuracy ofpositioning of the tip 313 can be raised and, because the fixing areaincreases, the strength of its fixing increased.

[0243] The structure of the tip 313 is shown in detail in FIGS. 54through 56.

[0244] Each cutting edge 316 formed on the tip 313 has an end cuttingedge 321, a thread-cutting edge 322 and a flat drag 323. The end cuttingedge 321 has a first end cutting edge 324 formed on the end of the tip313, a second end cutting edge 325 formed from the first end cuttingedge 324 toward the shank 311, and a third end cutting edge 326 formedfrom the second end cutting edge 325 further toward the shank. Thethread-cutting edge 322 is formed continuing from the third end cuttingedge 326 toward the shank. The flat drag 323 is formed continuing fromthe thread-cutting edge 322 further toward the shank. The referencenumeral 327 denotes a rake face and 328 a side flank. θ is the tip angleof the cutting edges 316 and is 180°.

[0245] In FIG. 55, the tip 313 consists of a three-layer laminatewherein a second layer 11 made of a hard sintered compact of CBN ordiamond is sandwiched between a first layer 12 and a third layer 13 madeof a tool material such as cemented carbide. When the cutting tool 310is seen from the front, the second layer 11 is a narrow band passingthrough the center of rotation of the tool, the cutting edges 316 areformed in this narrow band, and the second layer 11 is reinforced onboth sides by the first layer 12 and the third layer 13. The referencenumerals 335, 335 denote oil passages and 336, 336 denote chip dischargegrooves.

[0246] For the tip 313, with reference to FIG. 1 and FIGS. 16 through18, a semi-completed tip is obtained from a three-layer laminate andthen this semi-completed tip is brazed to the end of the shank 311 withthe brazing filler metal brazing filler metal 312. The cutting edges 316each made up of an end cutting edge 321, a thread-cutting edge 322 and aflat drag 323 are then formed on the semi-completed tip 316 to completea cutting tool 310 for thread-cutting.

[0247] As is clear from the detail view of FIG. 56, in the cutting edges316, 316, first end cutting edges 324, 324, second end cutting edges325, 325 and third end cutting edges 326, 326 continuous in the form ofa stairway, for generating chips finely, are formed; thread-cuttingedges 322, 322 for cutting a thread are formed; and flat drags 323, 323of the same diameter as the internal diameter of the female thread areformed. The reference numeral 338 denotes front flanks.

[0248] Next, an operation for cutting a thread using the thread-cuttingtool shown in the preferred embodiment of FIGS. 54 through 56 will bedescribed.

[0249]FIGS. 57A through 57D show a case wherein a cored hole 342 hasbeen provided in a casting 341 in advance. Forming a cored hole 342 inadvance like this promotes efficiency of machining.

[0250]FIG. 57A: The cutting tool 310 is brought to face the casting 341and, first, machining of a chamfer on the cored hole 342 is started.

[0251]FIG. 57B: With the cutting tool 310 rotating, the center axis 344of the cutting tool 310 is offset by a distance δ1 from the hole center343. The entrance of the cored hole 342 is cut by the end cutting edges321 and the thread-cutting edges 322, and a chamfer 345 is formed.

[0252]FIG. 57C A prepared hole 346 is formed by the end cutting edge 321and the thread-cutting edge 322. The internal diameter of the preparedhole 346 is substantially the same as the diameter of the thread-cuttingedges 322. Because the first end cutting edges 324, the second endcutting edges 325 and the third end cutting edges 326 for making thechips 347 small are formed in the cutting edges 316, even if the area ofthe chip discharge grooves 336 are made small, chips 347 can bedischarged easily. And by making the area of the chip discharge grooves336 small like this, it is possible to raise the rigidity of the cuttingtool 310. When the rigidity of the tool is high, an efficient cuttingcondition can be set, and productivity can be increased. When apredetermined depth is reached, feeding (lowering) of the cutting tool310 is stopped and thread-cutting is started.

[0253]FIG. 57D: With the cutting tool 310 rotating about its own axis,the cutting tool 310 is revolved to effect thread-cutting.Thread-cutting is carried out in the form of a spiral so that the tooladvances by a pitch P with each revolution. When thread-cutting ends,the cutting tool 310 is removed and a female thread 348 is obtained.When the cutting tool 310 is removed from the threaded hole, because thechamfer 345 has already been formed, there is no burring at the entrance351 of the threaded hole, and the labor of removing burrs can bedispensed with. Also, because the flat drags 323 the same diameter asthe internal diameter of, the female thread are formed on the cuttingedges 316, a crest of the ridge of the female thread having apredetermined internal diameter can be formed with the flat drags 323.

[0254] In this way it is possible to carry out formation of a chamfer,formation of a prepared hole, thread-cutting and formation of a threadridge crest with the single cutting tool 310. Consequently, there is noneed for a tool change and cutting work can be carried out continuously,without stopping.

[0255]FIGS. 58A through 58D show a process for cutting a thread in acasting 361 which does not have a cored hole.

[0256]FIG. 58A: The cutting tool 310 is brought to face the casting 361,and machining of a chamfer is started.

[0257]FIG. 58B: The machining of the chamfer is the same as describedwith reference to FIG. 57B. That is, the center axis 344 of the cuttingtool 310 is offset-by a distance δ1 and a chamfer 345 is formed by theend cutting edges 321 and the thread-cutting edges 322.

[0258]FIG. 58C: Next, the cutting tool 310 is lowered and machining of aprepared hole is carried out. The prepared hole machining is the same asdescribed with reference to FIG. 56C. Because the first end cuttingedges 324, the second end cutting edges 325 and the third end cuttingedges 326 for making the chips 347 small are formed in the cutting edges316, even if the area of the chip discharge grooves 336 are made small,chips 347 can be discharged easily. When the cutting tool 310 reaches apredetermined depth, feeding (lowering) of the cutting tool 310 isstopped and thread-cutting is started.

[0259]FIG. 58D: Thread-cutting also is the same as described withreference to FIG. 57D. That is, thread-cutting is effected by means ofthe thread-cutting edges 322 while the cutting tool 310 is lifted fromthe bottom of the prepared hole, and a female thread 348 is formed.

[0260] Thus, even without a cored hole in the casting, in the same wayas when there is a cored hole, the machining of a chamfer, the formationof a prepared hole and thread-cutting can be carried out with the singlecutting tool 310, and consequently the labor of changing tools iseliminated and productivity is increased.

[0261] Although in this preferred embodiment an example was used whereinthe point angle θ of the cutting edges 316 is 180°, as shown in FIG. 54,in the invention there is no limitation to this, and the point angle θcan be determined freely.

What is claimed is:
 1. A cutting tip comprising: a first layerconsisting of a cemented carbide tool material; a second layerconsisting of a hard sintered compact of CBN or diamond; and a thirdlayer consisting of a cemented carbide tool material, wherein the secondlayer is sandwiched with the first layer and the third layer to form athree-layer laminate, and a cutting edge is formed in the second layer.2. A method for manufacturing a cutting tip, comprising the steps of:preparing a three-layer laminate wherein a second layer consisting of ahard sintered compact of CBN or diamond is sandwiched by a first layerand a third layer consisting of a tool material such as cementedcarbide; cutting out a prismatic blank of rectangular cross-section bycutting the first layer, the second layer and the third layer in ordersubstantially perpendicularly from the upper face of the first layer;cutting out a semi-completed tip including the second layer in themiddle thereof by cutting from one cut face of the prismatic blank tothe other cut face; and obtaining a completed tip by forming on thesemi-completed tip a rake face, a cutting edge and a flank.
 3. A cuttingtool made up of a shank and a tip attached to the shank, wherein: thetip is made from a three-layer laminate wherein a second layerconsisting of a hard sintered compact of CBN or diamond is sandwiched bya first layer and a third layer consisting of a tool material such ascemented carbide; and when the cutting tool is seen in front view, thesecond layer is a narrow band passing through the center of rotation ofthe tool, a cutting edge is formed in this narrow band, and the secondlayer is reinforced on both sides by the first layer and the thirdlayer.
 4. A cutting tool according to claim 3, wherein an oil passage isformed in the shank and an oil passage is formed in the second layer ofthe tip and the two oil passages connect in a straight line.
 5. A methodfor manufacturing a cutting tool made up of a shank and a tip attachedto the shank, comprising the steps of: preparing a three-layer laminatewherein a second layer consisting of a hard sintered compact of CBN ordiamond is sandwiched by a first layer and a third layer consisting of atool material such as cemented carbide; cutting out a prismatic blank ofrectangular cross-section by cutting the first layer, the second layerand the third layer in order substantially perpendicularly from theupper face of the first layer; cutting out a semi-completed tipincluding the second layer in the middle thereof by cutting from one ofthe cut faces of the prismatic blank to the other cut face; joining thesemi-completed tip to a separately prepared shank; and obtaining acompleted tip by forming on the semi-completed tip a rake face, acutting edge and a flank.
 6. A manufacturing method according to claim5, wherein an oil passage is made in the shank and an oil passage ismade in the semi-completed tip and in the joining step thesemi-completed tip is positioned with respect to the shank by a pinbeing passed through the two oil passages and joining of thesemi-completed tip and the shank is carried out in this state.
 7. Amethod for manufacturing a cutting tip, comprising the steps of:preparing a three-layer laminate wherein a second layer consisting of ahard sintered compact of CBN or diamond is sandwiched by a first layerand a third layer consisting of a tool material such as cementedcarbide; cutting out a prismatic blank of rectangular cross-section bycutting the first layer, the second layer and the third layer in ordersubstantially perpendicularly from the upper face of the first layer;cutting out a semi-completed tip including the second layer in themiddle thereof by cutting the prismatic blank on a cutting planeorthogonal to or inclined at a predetermined angle to a cut face formedin the blank cutting step; and obtaining a completed tip by forming onthe semi-completed tip a rake face, a cutting edge and a flank.
 8. Amethod for manufacturing a cutting tool made up of a shank and a tipattached to the shank, comprising the steps of: preparing a three-layerlaminate wherein a second layer consisting of a hard sintered compact ofCBN or diamond is sandwiched by a first layer and a third layerconsisting of a tool material such as cemented carbide; cutting out aprismatic blank of rectangular cross-section by cutting the first layer,the second layer and the third layer in order substantiallyperpendicularly from the upper face of the first layer; cutting out asemi-completed tip including the second layer in the middle thereof bycutting the prismatic blank on a cutting plane orthogonal to or inclinedat a predetermined angle to a cut face formed in the blank cutting step;joining the semi-completed tip to a separately prepared shank; andobtaining a completed tip by forming on the semi-completed tip a rakeface, a cutting edge and a flank.
 9. A thread-cutting tool made up of ashank and a tip attached to the shank, wherein: the tip is made from athree-layer laminate wherein a second layer consisting of a hardsintered compact of CBN or diamond is sandwiched by a first layer and athird layer consisting of a tool material such as cemented carbide: andwhen the thread-cutting tool is seen in front view, the second layer isa narrow band passing through the center of rotation of the tool, an endcutting edge and a thread-cutting edge are formed in this narrow band,and the second layer is reinforced on both sides by the first layer andthe third layer.
 10. A thread-cutting tool according to claim 9, whereina flat drag of smaller diameter than the thread-cutting edge and largerdiameter than the shank is formed on the side of the tip attached to theshank, and the end cutting edge, the thread-cutting edge and the flatdrag are formed in this order in the second layer.
 11. A thread-cuttingtool according to claim 9, wherein an oil passage is formed in the shankand an oil passage is formed in the second layer of the tip and the twooil passages connect in a straight line.
 12. A method for cutting athread using a thread-cutting tool, comprising the steps of: making aprepared hole of substantially the same diameter as the externaldiameter of the thread-cutting tool by passing the thread-cutting toolinto a cored hole while rotating it about a threaded hole axis;offsetting the axis of the thread-cutting tool from the threaded holeaxis by a predetermined distance after the end of the thread-cuttingtool reaches the bottom of the prepared hole and starting thread-cuttingin the prepared hole with a thread-cutting edge formed on the tool; andcutting a thread with the thread-cutting edge by rotating the axis ofthe thread-cutting tool about the threaded hole axis while graduallywithdrawing the thread-cutting tool in correspondence with the lead ofthe thread.
 13. A method for cutting a thread using a thread-cuttingtool, comprising the steps of: cutting a thread in a cored hole with athread-cutting edge by offsetting the axis of the thread-cutting toolfrom the threaded hole axis by a, predetermined distance and thenturning the thread-cutting tool about the threaded hole axis androtating the thread-cutting tool and advancing the thread-cutting toolin correspondence with the lead of the thread; dragging with a flat dragformed on the thread-cutting tool the crests of the ridges of the threadcut out with the thread-cutting edge; and finishing with an end cuttingedge formed on the thread-cutting tool the bottom of the threaded hole.14. A thread-cutting tool made up of a shank and a tip attached to theshank, wherein: the tip is made from a three-layer laminate wherein asecond layer consisting of a hard sintered compact of CBN or diamond issandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide; and when the thread-cutting tool isseen in front view, the second layer is a narrow band passing throughthe center of rotation of the tool, a drill edge and a thread-cuttingedge are formed in this narrow band, and the second layer is reinforcedon both sides by the first layer and the third layer.
 15. Athread-cutting tool according to claim 14, wherein at least two oilpassages are provided in the shank and at least one oil passage isprovided in each of the first layer and the third layer of the tip sothat the oil passages in the shank and the oil passages in the tipconnect.
 16. A method for cutting a thread using a thread-cutting tool,comprising the steps of: making a prepared hole in a workpiece with adrill edge formed on a thread-cutting tool by rotating thethread-cutting tool about a threaded hole axis; offsetting the axis ofthe thread-cutting tool from the threaded hole axis by a predetermineddistance after the thread-cutting tool reaches the bottom of theprepared hole for starting thread-cutting in the prepared hole with athread-cutting edge formed on the thread-cutting tool; and cutting athread with the thread-cutting edge by rotating the axis of thethread-cutting tool about the threaded hole axis while graduallywithdrawing the thread-cutting tool in correspondence with the lead ofthe thread.
 17. A method for cutting a thread using a thread-cuttingtool, comprising the steps of: making a prepared through hole in aworkpiece with a drill edge formed on a thread-cutting tool by rotatingthe thread-cutting tool about a threaded hole axis; chamfering an outletof said prepared through hole with a back of said drill edge; offsettinga center axis of the thread-cutting tool from a threaded hole axis by apredetermined distance for cutting a thread in said prepared throughhole with a thread-cutting edge formed on said thread-cutting tool; andcutting a thread in said through hole with said thread-cutting edge bygradually pulling said thread-cutting tool out from said hole incorrespondence with a lead of said thread while rotating the axis ofsaid thread-cutting tool about said threaded hole axis.
 18. Ahole-finishing drill made up of a shank and a tip attached to the shank,wherein: the tip is made from a three-layer laminate wherein a secondlayer consisting of a hard sintered compact of CBN or diamond issandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide; when the drill is seen in front view,the second layer is a narrow band passing through the center of rotationof the tool, a cutting edge is formed in this narrow band, and a pair oflands are formed on the periphery of the tip; the cutting edge is astepped edge formed with a plurality of steps in the form of a stairwayradially outward from the drill center; and guide pads for preventingrun out of the tip are formed projecting on the lands of the drill. 19.A cutting tool made up of a shank and a tip attached to the shank,wherein: the tip is made up of a three-layer laminate wherein a secondlayer consisting of a hard sintered compact of CBN or diamond issandwiched by a first layer and a third layer consisting of a toolmaterial such as cemented carbide; when the cutting tool is seen infront view, the second layer is a narrow band passing through the centerof rotation of the tool, and a cutting edge is formed in this narrowband; and an end cutting edge constructed in the form of a stairway tokeep chips small, a thread-cutting edge for cutting a thread, and a flatdrag the same diameter as the internal diameter of the female thread areformed in the cutting edge in this order from the end of the tip towardthe shank.